The Power of Scheduling as an Energy Conservation Strategy

The graph below compares energy consumption on a recent project for a typical day starting with the pattern that existed before we started and ending with the current pattern. I have included it here because it shows how powerful scheduling can be in terms of generating savings, especially if it is combined with optimizing the performance of utility systems to match the load requirements.

The kW data is from the 480 volt distribution system, which is dedicated to the HVAC equipment. The HVAC equipment consists of two constant volume reheat air handling systems of approximately 25 kW each and a 100 ton air cooled chiller that can draw in excess of 100 kW fully loaded. There are also another 5-10 kW of minor loads like pumps and research equipment on the 480 volt system.

One of the air handling units is 100% outdoor air (basically, a Make-p Air Unit or MAU, just like this blog string is focused on), providing make up air to research and teaching labs. The other air handling unit is economizer equipped and serves office areas. The chiller has approximately 50% reserve capacity for redundancy and a future cooling load so it tends to never run more than 50% loaded.

The red line is the typical profile for an early September day in the Bay Area before we started. Both air handling systems ran round the clock and, because they were constant volume reheat system, the chiller tended to run with one compressor at full load. The small spikes are the chiller unloaders modulating capacity between 75% and 100%.

2009 – 2011 Changes

The green line represents the savings we achieved by making a very simple change in late 2009. We applied a schedule to the air handling unit that serves the office areas. The nominal 32 kW base load that exists overnight represents the operation of the MAU serving the labs along with the minor equipment. Note that because the AHU we applied the schedule to was a constant volume reheat system, we also saved significant thermal energy via a reduction in the reheat load.

That change was augmented with a programming change that implemented a flywheel cycle for the chiller during low load conditions. That change was made to protect the chiller from short cycling rather than to save energy. The low load condition occurred when a small research load was added that required round the clock chiller operation. The flywheel cycle changed the compressor operating pattern from 10-15 starts an hour (not good at all for compressor life) to one 10-15 minute on cycle followed by a 90-120 minute off cycle.

Once we had the chiller operating safely at low load, we realized that we could allow the air handling unit to operate on an integrated economizer cycle, which had the potential to open the door to additional energy savings.

In the typical 2o11 profile, the jump from a nominal 30 to 60 kW represents the start-up of the other air handling unit. The jumps from 60 kW to a nominal 100 – 120 kW and back represent the operation of the chiller once it became warm enough to cause it to operate. The large spikes during the unoccupied hours represent the chiller operating during the flywheel cycle.

The fact that the spikes during the unoccupied hours were so large and the fact that the chiller tended to run at full load even with an integrated economizer process surprised us and caused us to dig in a bit to understand what was going on. In doing that we discovered to energy savers.

The compressor hot gas bypass system was out adjustment, causing the compressors to run at nearly full load no matter what the actual load was. Fixing this saved thermal energy.

The chilled water valve in the office air handling unit had a stroke restriction applied to it that kept it from completely closing. This caused the system to over-cool and then compensate with extra reheat. Fixing this saved electrical and thermal energy.

An important point to realize here is that we may not have realized these to opportunities existed had we not been watching the facility’s energy use patterns consistently and had we not thought about what we expected and investigated when things didn’t make sense. Implementing the schedule played a role in exposing the issues too because it pushed the system into an operating regime it had not experienced before.

2011 – 2012 Changes

The changes between the 2011 (green line) and 2012 (blue line) consumption patterns can be attributed to three primary mechanisms.

The shift from a base load electrical consumption of a nominal 32 kW to a nominal 18 kW was created by implementing zone level scheduling for the MAU. Since the research labs were the only area that needed round the clock support we added a damper that allowed us to shut down air flow to the teaching labs when they were not in use and then added control logic to slow the main fan down as required to follow the load reduction. We also added dampers to a couple of conference areas and triggered them based on the occupancy sensor in the conference room that controlled the lights.

The reduction in the frequency and size of the peaks associated with the operation of the chiller resulted from having the manufacturer send a factory technician out to perform a service call and re-tune things like the hot gas bypass control system. We also worked with the factory to make some minor modifications to the chiller control system that allowed us to limit the number of compressors that ran during the flywheel cycle and limit their capacity, further optimizing that process.

The curve shape that is starting to emerge for the kW profile during the occupied hours is related to an experimental control strategy we implemented in the office air handling unit, essentially controlling it as a large, single zone VAV system. The approach seemed to work fairly well during the summer and swing season months. But we are still fine tuning it to achieve better comfort during the cold weather.

The General Case

The opening case study demonstrates the obvious; things that run round the clock will use more energy than things that can be scheduled. But some facilities like the case study facility and the hotel we are focused on in this string of blog posts require round the clock operation to meet their core mission.

When you are dealing with a facility that needs to run systems round the clock to meet its core mission (for the case study system, safe, quality research and for our hotel, guest satisfaction), a number of things can happen that can waste energy.

Utility systems supporting the system that provides the round the clock service need to run round the clock

When an end-use system runs round the clock, then the utility systems supporting it must also run round the clock. As a result, the utility systems can see a large variation in load between the peak that occurs with everything operating at design conditions and the valley that occurs with one or two systems running at a very low load condition.

Frequently, there are opportunities to optimize the way utility systems operate to serve the end-use loads by understanding the load profile the loads create and then adjusting the utility system so that it can follow it in an efficient manner. For example, using trend data to develop a load profile for a building (an RCx technique) and then purchasing a chiller with a turn-down capability to match the profile and sized for the point on the profile where the most hours occur (vs. the design condition, where hardly any of the hours of actual operation occur) can have a significant impact on operating costs.

It’s easier just to run everything round the clock

If you are a typical facility operator these days, you probably have more to do than time to do it given staffing cuts and the demand for maintaining more sophisticated technology with a shrinking maintenance budget. Lacking a centralized automation system, operators who are spread too thin simply may not have time to start-up and shut down equipment to match a variable schedule of use.

This can be especially true in a Hotel, where the schedules for meeting rooms and related support areas can vary from hour to hour and day to day. And even if you have an automation system, if changing schedules is a multi-step process on a slow running system, you may be inclined to put in a default that covers all of the bases (for instance, run everything from 6 am through 9 pm) rather than try to modify it daily.

And, if something goes wrong with that strategy, for instance;

A meeting runs late, the system shuts down, and the guest complains and is compensated financially for their discomfort, a cost that by far exceeds the energy savings, at least in management’s eyes, or

Extreme weather causes a meeting room not to be at the desired operating temperature by the time the occupants show up and the same thing happens;

You may be tempted (or simply told by management) to keep everything running round the clock.

Playing into this is access; I was just in a facility where to get to the motor control center for most of the air handling systems, you had to climb up a 20 or so foot long vertical steel ladder with no safety cage and a long step from the ladder to the equipment room floor. The facility had no automation system and no time clocks and the operators had to make the trip up the ladder anytime a system needed to be started or stopped.

If I was tired or had a sore back (or was afraid of heights), I might be sorely tempted to just let things run every once-in-a-while.

It’s safer just to run everything round the clock

This is a variation on the preceding theme but related to the risk associated with not having a space under control by the time it is supposed to be occupied, or worse yet, freezing a plumbing line or developing a condensation problem in an extreme climate. A central automation system can help, but if your facility has highly variable schedules and scheduling is a painful process in your control system, you may elect to simply let everything run, especially during extreme weather.

One zone on a multiple zone system runs round the clock, so the entire system runs round the clock

It is not uncommon for a requirement to maintain conditions in one zone on a multiple zone system to cause the operating team to operate the entire system on a 24/7 basis. Several years ago, I did some work on a large (800,000 square foot) office complex were all 23 air handling systems ran 24/7 due to the need to maintain conditions in a hand-full of zones that were scattered amongst them.

We were able to save considerable energy and other resources by implementing schedules at the zone level instead of the system level. This means that if a zone was not in use, we modified the terminal unit control strategy to simply shut down air flow and allow the space temperature to drive between limits. When the terminal unit shut down, the central system responded just like it would if any zone reduced its demand.

We were able to accomplish this via programming changes because the terminal units were DDC. But I have accomplished a similar strategy on non-DDC equipped systems or on systems that did not have some sort of volume regulation at the zone level by installing two position dampers in the duct serving the zone(s) I wanted to shut down and the using a schedule to control the dampers.

Our Target Facility’s Case

In the of our hotel in Golden Colorado, the guest rooms and some support areas have fan coil units. Depending on the exposure of the space served by the fan coil unit, the internal loads, and the requirements of the occupants, you can require cooling or heating just about any day of the year.

For instance, in the winter, a room with a well insulated small square footage exterior wall containing a window facing south on a sunny day may require cooling, even thought its cold outside if the occupant is to remain satisfied. Remember, from the hotel’s perspective, the occupant is a guest and ultimately what the hotel is selling is guest satisfaction.

In a similar vein in the middle of the summer, you could be dealing with what hotel operators call the little old lady who likes her room 80 (as in 80°F). So, hot water is required even during warm weather.

The bottom line is that for most hotels with central plants, it is necessary to provide chilled and hot water 24/7 to ensure guest satisfaction. The trick is to figure out how to do that as efficiently as possible.

Because heating and cooling are available round the clock, things are set up for systems like the MAU we are focused on to use both utilities concurrently, even if they don’t need to. That is one of the things that makes a MAU in a facility that runs round the clock a good RCx target, even if the system itself is operated on a schedule.